Recent months have underscored the need for effective strategies for circumventingthreats from biological weapons. Preventive vaccination, while ideal, presupposes a known pathogen and well-developed vaccinationprotocols, both of which are problematic for logistical, scientific, and ethical reasons. It is clear, however, that immunization is the single most effective way to prevent pathogen mediated disease, but it is also clear that this approach is fraught with gaps in our knowledge of the human immune response as well as pathogen biology. For reasons outlined in the introduction to this center grant, we focus on influenza virus as an example only partly because of its potential as a bioweapon and mostly because antigenic shift and drift inviral components lead to major disease epidemics, influenza virus has also been studied extensively at the structural level and most individuals have been exposed either environmentally or through vaccination. This enables the possibility of studying peripheral T-cell responses in the blood of volunteers. While Project 1 focuses upon class I restricted T-cell responses, this project will consider responses by helper (Th) T-cells, which are restricted by HLA class II molecules. Th-cells are critical in the defense against influenza virus because they drive B-cell production of neutralizing antibodies and development of cytolytic T-cells (CTL) through their production of immunoregulatory cytokines. Our previous work has demonstrated a number of important epitopes in the influenza matrix protein (MP) and hemagglutinin (HA) antigens of the virus. One of these comprises the immunodominant epitope HA306-320 in the HA1 chain of an H3N2 strain (A/Texas/1/77). While much is known about H3, relatively little is known regarding the epitopes of other variants of influenza virus, for example, the H1 and H2 strains. Further, essentially nothing is known about the complexity of the helper T-cell response to these antigens, neither is it known how stable the helper T-cell repertoire may be in the human population. Similarly, the extent of cross-reactivity amongst these different strains has received little attention. These facts are important on epidemiological, vaccine development, and biodefense grounds. It is our hypothesis that by first understanding these issues, we may be able to develop vaccine approaches that would take advantage of elements of cross-reactivity to elicit cross-protection amongst influenza virus strains. This hypothesis will be examined in human volunteers screened for responses to influenza virus in vitro and in HLA transgenic mice in vivo as enumerated in the following specific aims: To identify the DR1-restricted epitopes; To determine the complexity of the Th response to DR1-restricted viral antigens; To determine the stability of the Th response; To evaluate whether murine T-cell responses correspond to those identified in patients under Aim 2.